In conventional continuous steel casting machines, molten steel is passed through a generally vertically oriented copper mold tube to form a cast steel strand or slab. The temperature of the molten steel is typically about 2850.degree. F., although with certain grades of metal the temperature may be as low as 2600.degree. F. In general, although most of the references herein are to steel casting, the invention contemplates the casting of any metal or metal alloy whose liquid temperature exceeds about 2600.degree. F. As the molten steel passes through the mold tube, its outer shell hardens. The hardened outer shell confines the molten steel core of the cast strand while the strand continues to cool and solidify following its exit from the mold tube.
The mold which forms the steel strand contains the liquid steel and provides for its initial solidification, that is, hardening of the outer shell. The solidifying strand is extracted continuously from the bottom of the mold at a rate equal to that of the incoming liquid steel at the top, the production rate of the mold being determined by the time required for the outer shell to harden sufficiently so as to contain the inner core of liquid steel as the strand exits the mold tube.
The mold tube and thus the liquid steel flowing therethrough are preferably cooled with sprays of coolant water directed against the outside of the mold tube during a casting operation. Since industrial water systems used for cooling purposes nearly always contain various amounts of minerals dissolved in solution, and since the mold tubes are very hot, when the coolant water comes into contact with the hot mold tube surface, some of the minerals in the water will precipitate onto the mold tube surface. The minerals most commonly present in the coolant water are compounds of calcium and magnesium.
This precipitate layer will build up over a period of time and act as an insulating barrier to heat transfer from the molten metal and into the coolant water being sprayed against the mold tube. A layer of precipitate as thin as 0.003 inch will reduce the rate of heat transfer through the mold tube wall by about 50-75%. The net effect of this reduced heat transfer rate is that the thickness of the solidifying steel shell will be markedly reduced, often resulting in the shell being too thin to support the ferrostatic pressure of the core of liquid steel as the cast strand leaves the mold tube. This thin shell can rupture (commonly referred to as a "break-out"), allowing molten steel to spill out, thereby causing considerable damage to machinery and creating a dangerous condition for machine operators. Poor steel quality also results.
The adhering strength of the precipitate layer to the mold tube wall is directly related to the temperature of the copper mold tube wall and the length of time that the precipitate layer remains undisturbed on the mold tube. It is thus desirable to clean the precipitate layer from the mold tube wall as quickly as practical after its formation, and preferably while the mold tube wall is still warm (about 210.degree. F.).
However, in conventional mold assemblies it is not possible to meet these conditions, since the mold tube wall is normally not accessible without disassembling the mold and removing the mold tube for cleaning outside of the machine. Because this is an expensive and time-consuming proposition, cleaning typically only takes place when the precipitate layer has built up and heat transfer through the mold tube wall has deteriorated to an unacceptable level. This, of course, increases the danger of a "break-out", slows production, and reduces the quality of the cast metal.
Moreover, since the design of a conventional continuous casting mold requires that a baffle tube be placed in close proximity to the cooling mold tube, it is not possible for the machine operators to visually inspect the mold for precipitate build-up without completely disassembling the mold and removing the copper mold tube. After the mold tube has been disassembled from a conventional mold, the tube is typically subjected to a grinding or polishing operation to remove the precipitate layer. This polishing operation can only be accomplished during a shutdown of operations and requires expensive labor and costly down-time in the steel plant.
There is thus need for a means and method to clean the precipitate from a mold tube in a continuous casting machine, without requiring disassembly of the mold.